Technique for sensing altitude from fan speed

ABSTRACT

A DC fan, or lot of DC fans is characterized at a constant voltage to determine the variation of their rotational speed with respect to altitude. Many such DC fans will have a substantially linear response in speed with respect to altitude. From this relationship, a converter is constructed to convert the rotational speed into an altitude. The converter may be a discrete electronic device including a look up table or capable of performing the arithmetic algorithm representing the relationship between fan speed and altitude. Alternatively, the converter may be incorporated into the system to be cooled by the DC fan. For example, it may be a software routine run by the computer that the DC fan is used to cool.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to application Ser. No. xx/xxx,xxx entitled, “Utilizing an Altitude Sensor to Control Fan Speed,” filed on or about the same date as the present application, and hereby incorporated herein by reference. Application Ser. No. xx/xxx,xxx discloses and claims a technique utilizing the altitude calculated from the fan speed in a method to set a fan speed sufficient to allow for proper processor thermal margin.

FIELD OF THE INVENTION

The present invention relates generally to the field of cooling technologies and more specifically to the field of cooling technologies within a device enclosure where cooling efficiency is related to fan speed and altitude.

BACKGROUND OF THE INVENTION

As altitude above sea level increases, atmospheric density decreases. This decrease in atmospheric density is responsible for a reduction in cooling capacity of a fan running at a given speed. Since there is less air at higher altitudes, at a given fan speed fewer air molecules will be passing over a heat-generating device, than would be present in the identical system at a lower altitude. This fact presents a problem for designers looking to characterize system requirements, since a given configuration that works well at sea level, may be sufficiently degraded in cooling capacity at higher altitudes such that some electronic devices may no longer be operating within their thermal design margins.

Designers have typically solved this problem by requiring sufficient cooling of all of their systems for performance at altitude. However, this solution is not optimum for systems operating at sea level, since the same system could operate at a higher frequency at sea level due to the improved air-cooling present at sea level. System performance could be maintained at all altitudes by requiring fans in high altitude systems to run faster, however this requires knowledge of altitude. While it is certainly possible to require users to input altitude information upon first use of a system, this approach is prone to errors. There is a need in the art for a method allowing electronic systems to detect their operating altitude so that they may respond accordingly.

SUMMARY OF THE INVENTION

A DC fan, or lot of DC fans is characterized at a constant voltage to determine the variation of their rotational speed with respect to altitude. Many such DC fans will have a substantially linear response in speed with respect to altitude. From this relationship, a converter is constructed to convert the rotational speed into an altitude. The converter may be a discrete electronic device including a look up table or capable of performing the arithmetic algorithm representing the relationship between fan speed and altitude. Alternatively, the converter may be incorporated into the system to be cooled by the fan, for example, it may be a software routine run by the computer that the DC fan is used to cool.

Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a DC fan and converter according to the present invention.

FIG. 2 is a graph showing the relationship between fan rotational speed and altitude in an example embodiment of the present invention.

FIG. 3 is a graph showing the relationship between fan rotational speed and processor thermal margin in an example embodiment of the present invention.

FIG. 4 is a flowchart of an example embodiment of the calculation of altitude from fan rotational speed according to the present invention.

DETAILED DESCRIPTION

FIG. 1 is a view of a DC fan and converter according to the present invention. In an example embodiment of the present invention a DC fan 100 including fan blades 104, a motor 102, and an electrical port 108 is provided to cool a heat-generating device. The DC fan 100 may have the ability to output its rotational speed from the fan 100 itself without any additional devices. Alternatively a speed sensor 106, such as an opto-electronic device that counts fan blades 104 may be used for DC fans 100 without the ability to output their rotational speed. The speed data from the fan 110, or the speed data from the speed sensor 112 is then input to a converter 114 that converts the speed data into altitude data 116. The converter 114 is programmed using data obtained by characterizing the rotational speed of the DC fan 100 with respect to altitude. While FIG. 1 shows a discrete converter device 114 for simplicity and clarity, other embodiments of the present invention may include the converter function in other electronic devices present in the overall device that is being cooled by the DC fan 100. For example, in a computer system cooled by the DC fan 100, the converter functionality may be built in to the processor chip, or may operate in software under the computer operating system. The physical location and construction of the converter 114 is not critical to the present invention, and the converter 114 functionality may be implemented anywhere desired by the system engineer. A sample of DC fan characterization data is shown in FIG. 2.

FIG. 2 is a graph showing the relationship between fan rotational speed and altitude in an example embodiment of the present invention. Since the atmosphere is less dense at altitude than at sea level, a DC fan 100 supplied with a constant power voltage will rotate at a higher rate at higher altitudes. An example graph of this relationship between rotational speed and altitude is shown in FIG. 2. In this example graph of a characterization of a DC fan 100, the horizontal axis 204 represents altitude above sea level, measured in feet, and the vertical axis 202 represents rotational fan speed, measured in revolutions per minute (RPM). In this example embodiment, the characterization data 200 is represented by a straight line. Naturally, most embodiments of the present invention will take fan speed data at a variety of atmospheric pressures related to a variety of altitudes and then a curve will be fit to the data. This curve may be linear in some cases, but other curves may be fit to the characterization data within the scope of the present invention.

Note that in this example characterization graph, at a first data point 214, the DC fan 100 rotates at 2500 RPM (represented by point 206 in FIG. 2), and at a second data point 216, when the DC fan 100 is at an altitude of 2000 feet (represented by point 208 in FIG. 2). At a higher altitude of 12,000 feet (represented by point 212 in FIG. 2), the DC fan rotates at 3000 RPM (represented by point 210 in FIG. 2). While this sample characterization data is linear, characterization of other DC fans 100 may result in non-linear characterization data within the scope of the present invention. This characterization data may be described by an arithmetic algorithm, a look up table, or other equivalent mechanisms or methods for calculation of an altitude when given a fan rotational speed. The resulting characterization data is then programmed into the converter 114 shown in FIG. 1.

FIG. 3 is a graph showing the relationship between fan rotational speed and processor thermal margin in an example embodiment of the present invention. As fan speed increases, the amount of air flowing over a heat-generating device also increases. This increased airflow results in more efficient cooling of the heat-generating device resulting in a lower temperature of the heat-generating device. This relationship is shown graphically in FIG. 3. In this example graph of the relationship between the temperature of a heat-generating device, the horizontal axis 304 represents the fan speed, measured in RPM, and the vertical axis 302 represents the temperature of the heat-generating device, shown as thermal margin in a processor, and measured in degrees Centigrade (degrees C.). Processor thermal margin is the temperature difference between the current temperature of the processor and the maximum allowed temperature. Lower actual temperatures of the heat-generating device result in larger thermal margins. The example thermal data 300 shown in FIG. 3 is represented by a straight line, however other embodiments of the present invention may result in non-linear thermal data.

Note that in this example thermal graph, at a first data point 314, at a fan speed of 2500 RPM (represented by point 308 in FIG. 3), the processor has a thermal margin of 1 degree C. (represented by point 306 in FIG. 3), and at a second data point 316, at a fan speed of 3000 RPM (represented by point 312 in FIG. 3), the processor has a thermal margin of 8 degrees C. (represented by point 310 in FIG. 3). Thus, for an increase in fan speed of 500 RPM the processor thermal margin increased by 7 degrees C., which may be critical to processor performance in some designs.

FIG. 4 is a flowchart of an example embodiment of the calculation of altitude from fan rotational speed according to the present invention. In an example embodiment of the present invention, a method of determining altitude from fan speed is begun at a start step 400. In a preliminary step 402 a DC fan, or a group of DC fans, is characterized to determine their response to altitude as measured by rotational fan speed at a constant input voltage. Note that in some embodiments of the present invention, it may not be necessary to characterize every individual DC fan. Process variations within a given model of fan may be sufficiently small that characterization of a sample of fans from that given model may be sufficient to generate characterization data usable by all fans of that model. In a step 404, a DC fan speed of a fan is detected. In a step 406, this fan speed is converted to an altitude by a converter, and the method ends in a finish step 408.

The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art. 

1. A device, comprising: a fan including a speed output, wherein a rotational speed of said fan is characterized with respect to altitude; and a converter electrically coupled to said speed output from said fan, wherein said converter receives a fan speed and outputs an altitude.
 2. The device of claim 1, wherein said converter uses an arithmetic algorithm to calculate said altitude from said fan speed.
 3. The device of claim 1, wherein said converter uses a look up table to calculate said altitude from said fan speed.
 4. The device of claim 1, wherein said fan speed is output by said fan as a digital signal.
 5. The device of claim 1, wherein said fan speed is output by said fan as an analog signal.
 6. A device, comprising: a fan, wherein a rotational speed of said fan is characterized with respect to altitude; a fan speed detector, outputting a fan speed; a converter, electrically coupled with said fan speed detector, wherein said converter receives said fan speed and outputs an altitude.
 7. The device of claim 6, wherein said converter uses an arithmetic algorithm to convert said fan speed to said altitude.
 8. The device of claim 6, wherein said converter uses a look up table to convert said fan speed to said altitude.
 9. The device of claim 6, wherein said fan speed is output by said fan speed detector as an analog signal.
 10. The device of claim 6, wherein said fan speed is output by said fan speed detector as an analog signal.
 11. A method for the determination of an altitude, comprising the steps of: a) characterizing a rotational speed of a fan with respect to altitude; b) measuring a rotational speed of said fan; and c) converting said rotational speed into an altitude.
 12. The method of claim 11, wherein said converting step is performed using an arithmetic algorithm.
 13. The method of claim 11, wherein said converting step is performed using a look up table.
 14. The method of claim 11, wherein said measuring a rotational speed of said fan step is performed by said fan.
 15. The method of claim 11, wherein said measuring a rotational speed of said fan step is performed by an optoelectronic device.
 16. A device, comprising: means for detecting the speed of a fan; and means for converting said speed of said fan into an altitude.
 17. The device of claim 16, further comprising: means for characterizing said speed of said fan with respect to altitude. 